This invention relates generally to semiconductor lasers and more particularly to a semiconductor laser of the type in which the radiation angle of an emission beam is turnable.
A so-called "phased-array semiconductor laser" which has a large number of multiple emission stripes and generates optical coupling betwen the stripes is a known example of high output semiconductor lasers (Tanetani et al., "32nd Proceeding of Japan Applied Physics Conference", p. 149, No. 1a-ZB-10 (April, 1985)). In this structure, however, a fundamental mode is more difficult to oscillate than a higher order mode as will be described elsewhere. Accordingly, it is known that oscillation occurs in the higher order mode and the radiation beam becomes two or more.
Since the oscillation mode in the phased-array semiconductor lasers is generally referred to as a "supermode", this specification will hereinafter use the same term.
Here, the outline of the supermode will be described, though its detail is described in E. Kapon et al., "OPTICS LETTERS", Vol. 10, No. 4, p. 125 (April, 1984).
FIGS. 2, 2a, 2b, 2c and 2d of the accompanying drawings shows schematically the relation between the refractive index and gain (absorption) in the phased-array semiconductor laser. In short, an optical gain exists in a region having a large refractive index such as an emission stripe portion and large optical absorption is generated in a region having a small refractive index such as in a gap between the emission stripes. A fundamental mode beam having an optical field such as shown in FIG. 2c has an optical field in a region having large absorption, and since this fundamental mode is absorbed by large absorption in the gap between the stripes, the fundamental mode beam has a large optical loss as a whole and hence, is difficult to oscillate. In other words, its threshold current is great. On the other hand, a higher order mode beam such as shown in FIG. 2d is likely to oscillate because the optical field does not substantially exist in the region having large absorption and no absorption of the optical field is generated in this region.
As can be seen from FIG. 2d, the phase of the optical fields of adjacent higher order mode beam are different from each other by 180.degree., and the emission beam having a radiation angle near 0.degree. offsets with each other and is distinguished so that the emission beam is distributed in a direction of the radiation angle .theta..sub.o (see FIG. 1(b)).
Incidentally, the radiation angle .theta..sub.o is given by the following formula EQU .theta..sub.o .apprxeq.Sin.sup.-1 (.lambda./2S)
where S in the gap between the stripes and .lambda. is the emission beam wavelength.
The prior art technique described above does not take into consideration any arrangement which makes the radiation angle of the laser beam turnable and the prior art semiconductor lasers are only desgined for high output semiconductor lasers.
In addition, electrodes in the conventional phased-array semiconductor lasers are not separated from one another but are continuous throughout the entire surface.